Washington: Floating bridge capitol of the world

By SCOTT GUTIERREZ, SEATTLEPI.COM STAFF

Updated
12:41 am PST, Wednesday, February 29, 2012

Original Lake Washington floating bridge: Opened on July 3, 1940, this was the concrete bridge that started them all in Washington state. The state's highway chief, Lacey V. Murrow, endorsed a concrete floating bridge despite intense skepticism. As former WSDOT chief bridge engineer Charles Gloyd wrote about the bridge's debut in a 1988 article, "certain wags wore life jackets to show their lack of faith."

Original Lake Washington floating bridge: Opened on July 3, 1940, this was the concrete bridge that started them all in Washington state. The state's highway chief, Lacey V. Murrow, endorsed a concrete floating

Original Lake Washington floating bridge: Opened on July 3, 1940, this was the concrete bridge that started them all in Washington state. The state's highway chief, Lacey V. Murrow, endorsed a concrete floating bridge despite intense skepticism. As former WSDOT chief bridge engineer Charles Gloyd wrote about the bridge's debut in a 1988 article, "certain wags wore life jackets to show their lack of faith."

Original Lake Washington floating bridge: Opened on July 3, 1940, this was the concrete bridge that started them all in Washington state. The state's highway chief, Lacey V. Murrow, endorsed a concrete floating

Washington state knows a thing or two about how to take a massive chunk of concrete and make it float.

The greater Puget Sound area is home to five of the world's floating highway bridges (four that are among the largest), which is no small feat considering only about a dozen or so are in operation around the globe.

If you're curious to see the state Route 520 Bridge's international cousins, the photo gallery above shows most of them -- structures made of concrete and steel that carry traffic over tricky geographical locations.

Soon, the state Department of Transportation will begin building the successor to the 520 bridge, already the longest floating span in the world, to give traffic a safer and less congested cross-lake trip between Seattle and Medina. The final project is budgeted at $4.6 billion.

Why did the state choose floating bridges to cross local waterways such as Lake Washington? It was a matter of cost and engineering in a deep, soft lake bed. The deepest point in Lake Washington is 214 feet, and the bridge's support towers "would have to be approximately 630 feet in height, nearly the height of the Space Needle, to support the bridge" according to WSDOT. That potentially would add billions to the price tag.

Pontoon bridges have been used for thousands of years in military and civilian applications, dating back as far as Xerxes' army campaigns in 480 B.C. in Europe. Xerxes' bridges were fashioned from 700 ships anchored together in two lines. Large cables of flax and papyrus were placed over the ships to support wood planks and a roadway with parapets built on top, according to a 1988 article by Charles S. Gloyd, former WSDOT chief bridge engineer, in Concrete International. (Gloyd noted that after the first two bridges were destroyed by storm, Xerxes had some of his engineers beheaded.)

In the 1930s, Seattle engineer Homer Hadley suggested that a concrete pontoon bridge might be feasible for carrying traffic across the depths of Lake Washington. Hadley had worked for a firm designing concrete barges during World War I and he proposed the idea of connecting hollow concrete barges end-to-end to Lacey V. Murrow, the state's then director of highways.

"Floating structures have been built for a long time. We were unique in that we were taking reinforced concrete and making that float," said Patrick Clarke, WSDOT's design manager for floating bridges and special structures. (A Popular Mechanics article explained how a floating bridge stays buoyant.)

While the first floating bridge initially met success after opening in 1940, there were epic setbacks. The bridge capsized during a severe storm in 1990. Prior to that, the Hood Canal Bridge, which opened in 1961, partially sank 18 years later in a storm. From those disasters, WSDOT learned lessons that led to improvements in design, which included several more decades of weather data that more accurately showed the full strength of the region's rain and windstorms. Those lessons have been incorporated into the design of the new state Route 520 Bridge.

"We're the only ones who have stuck with it and been successful for a long period of time," Clarke said. "I think we have worked out a lot of the bugs and they work really well for our application."

Over the last few decades, engineers from Canada, Norway, Japan and other countries have traveled to Washington to study how the state's floating bridges were built and maintained. In Norway, two pontoon highway bridges have been constructed to link communities across two of the country's fjords that reach 1,500 feet in depth.

In the above gallery, compare the size and design of various bridges, as well as the picturesque landscapes where they were built. The gallery also includes photos showing the evolution of floating structures in Washington state, as well some lesser known buoyant bridges.